We have synthesized malonaldehyde uniformly labeled with C14. In the course of this work we discovered that the common procedure for the preparation of malonaldehyde (i.e., hydrolysis of tetraalkoxypropanes) produced appreciable concentrations of side products. Since this is a method which was used for the estimation of the mutagenicity and carcinogenicity studies the possibility exists that malonaldehyde is not mutagenic and carcinogenic but that one of the side products of the preparation was responsible for this behavior. We are in the process of determining whether pure malonaldehyde is a mutagen and carcinogen. We plan to use our radiolabeled material to elucidate the pathways for the metabolism of malonaldehyde. Since covalent binding to nucleic acids and proteins is often suggested as an important step in the development of neoplastic states, we describe experiments designed to estimate the relative rates of macromolecular binding and matabolism in vivo and in vitro. These experiments and others will serve as a basis for our long term study of the physiological role of malonaldehyde and its involvement in the evolution of pathological states.